This invention relates to a prosthesis, particularly a prosthesis suitable for replacement of a metacarpophalangeal joint, i.e a knuckle joint.
Damage may be sustained by the joints in the hand due to various reasons, but the most common are osteoarthritis and rheumatoid arthritis. Osteoarthritis occurs as a result of injury to the hands and is specific to an affected area, i.e. where the damage occurred. The finger joints have no protection, unlike the knee joint which is protected by the knee cap, and therefore the articular surfaces may be easily damaged through injury.
Rheumatoid arthritis is a progressive, debilitating disease which attacks many joints in the body. A typical indication of rheumatoid arthritis is the occurrence of ulnar drift, where the metacarpophalangeal joint suffers particularly badly. This occurs because rheumatoid arthritis also significantly affects the soft tissue and tendons in the body which compounds the effects of the disease on the knuckle joints.
Some different aspects of hand function are described in an article xe2x80x9cA biomechanical analysis of the metacarpophalangeal jointxe2x80x9d by N. Berme et al, J. Biomechanics, 1977, Vol. 10, pages 409 to 412.
Various prostheses have been proposed in the past for replacement of the metacarpophalangeal joint. Early prostheses incorporated a constrained steel-hinge mechanism. Later designs utilise separate metacarpal and phalangeal components. For example, U.S. Pat. No. 3,946,445 describes a prosthesis comprising two components each having a rounded bearing body and an integrally projecting intramedullary stem. Ribs and grooves on the two components intermesh for component engagement and the metacarpal component body overhangs the stem at a concave rib end in order to provide knuckle simulation.
U.S. Pat. No. 5,007,932 describes an artificial bone joint suitable for replacing a knuckle joint having anchor portions fitted into the bone comprising recesses or bosses. These recesses or bosses face the direction of insertion of the anchor portion and form an angle of less than 100 degrees. This is said to overcome the long period of time that was previously required to allow embedding of the joint structure in the patient""s articulation.
International Patent Publication No. WO 95/09587 describes a joint prosthesis which allows rotary circumduction and, therefore, is said to be particularly adapted for replacement of the thumb joint. The two components comprise saddle shaped articulating surfaces which bear upon each other in such a way as to allow the bones to move in rotary circumduction with respect to each other.
International Patent Publication No. WO 95/09588 describes a prosthesis for the replacement of joints between long bones in the hand, including metacarpophalangeal joints, comprising two components each having an elongate stem for reception in one of the elongate bones and each having lateral articulating surfaces. The lateral articulating surfaces engage and articulate with each other such that lateral articulation is restrained only when the bones are articulated through at least 30 degrees of flexion. Parts of the metacarpal and phalangeal bones must be removed in order to fit the prosthesis which is not fixed by bone cement. No provision is made in this design of prosthesis to prevent ulnar drift.
Historically interposition arthroplasty has proven to be haphazard. It is rare that full flexion and extension is achieved once a metacarpophalangeal prosthesis is fitted and bone erosion, even breakage, is a significant problem. When the prosthesis is not fixed into the bone it may flex and piston in and out of the bone which presents the opportunity for significant wear. Moreover loosening of prostheses used in the fingers is a problem in the same way as in other total joint replacements such as hip, knee and shoulder replacements. It has been postulated that the problem lies with the means of fixation of the prosthesis and that the subsidence which is seen in the hip joint is also a problem with metacarpophalangeal and proximal interphalangeal joints. Often the prosthesis migrates down the intramedullary canal.
In an article entitled xe2x80x9cThe development, technique, and early clinical results of total joint replacement for the metacarpophalangeal joint of the fingersxe2x80x9d, by Arthur D. Steffee et al., Orthopedics, February 1981, Vol 4/No. 2, pages 175 to 180, there is described a prosthesis which was developed with the aim of overcoming recurrent problems of ulnar drift and extensor lag. It is suggested that the addition of xe2x80x9cwingsxe2x80x9d laterally on the plastic socket part of the prosthesis may help with alignment of the prosthesis in the canal, amongst other things.
A one piece prosthesis has been marketed under the name Silastic(copyright) HP 100 Swanson Finger Joint Implant. This is further described, for example, in U.S. Pat. No. 3,875,594.
In many cases the surgeon will decide to replace all of the metacarpophalangeal joints of a patient""s hand. In order that the surgical procedure shall not be too extended, it is recognised that the time to replace each individual joint should desirably be as short as possible. Moreover the joint should mimic as nearly as possible the action of the natural joint.
There is a need in the art for an improved prosthesis for replacement of the metacarpophalangeal joint. In particular there is a need for a metacarpophalangeal joint prosthesis which can be surgically inserted in a short space of time. In addition there is a need for a prosthesis for a metacarpophalangeal joint which can be inserted without use of bone cement.
It is accordingly an object of the current invention to provide an improved prosthesis for metacarpophalangeal joint replacement. A further object is to provide a prosthesis for replacement of the metacarpophalangeal joint which prevents wear and potential breakage of bone while providing full flexion and extension of the joint.
According to the present invention there is provided a prosthesis for a metacarpophalangeal joint comprising:
a metacarpal plug member for reception in a surgically prepared bore in a metacarpal bone comprising an elongate body which generally tapers from a proximal end to a distal end, which has a central longitudinal axis extending from the proximal end to the distal end, and which is formed with an axial bore extending from the proximal end towards the distal end, and a plurality of axially spaced first fins projecting from the elongate body in a direction substantially perpendicular to the central longitudinal axis of the elongate body for locating the metacarpal plug member in the surgically prepared bore in the metacarpal bone;
a metacarpal insert member comprising a head portion having a convex articulating surface and a stem portion projecting distally from the head portion for reception in the proximal end of the axial bore of the metacarpal plug member; and
a phalangeal insert member for reception in a surgically prepared bore in a proximal phalangeal bone comprising an enlarged proximal end portion and an elongate shaft portion that extends distally from the enlarged proximal end portion, the enlarged end portion being provided with a concave articulating surface substantially congruent with the convex articulating surface of the metacarpal insert member, and the elongate shaft portion having a longitudinal axis, tapering generally towards a distal end of the phalangeal insert member, and being provided with a plurality of axially spaced second fins projecting therefrom in a direction substantially perpendicular to the longitudinal axis thereof for locating the phalangeal insert member in the surgically prepared bore in the proximal phalangeal bone.
Preferably the metacarpal plug member is formed from a biocompatible plastics material. Also the phalangeal insert member is preferably formed from a biocompatible plastics material. A suitable biocompatible plastics material is ultra high molecular weight polyethylene.
The metacarpal insert member is preferably formed from a biocompatible metal material. Suitable biocompatible metal materials include cobalt-chrome, titanium, titanium alloys, titanium coated cobalt-chrome, and stainless steel.
The first fins may be circular in shape but are preferably non-circular in shape. Hence in a preferred design of prosthesis according to the invention the first fins are substantially triangular in shape. They may have the corners of the triangular shape somewhat cropped so that the first fins have the shape of an irregular pentagon. In one design the metacarpal plug member is provided with four first fins. Typically the first fins are about 0.6 mm thick.
Suitably the diameter of the axial bore in the metacarpal plug member is about 3.0 mm.
The metacarpal plug member may have a dorsal chamfered face adjacent its proximal end which is adapted to mate with a corresponding surface on the distal side of the head portion of the metacarpal insert. In this case the dorsal chamfered face makes an angle of about 35xc2x0 with the central longitudinal axis of the metacarpal plug member.
The convex articulating surface of the metacarpal insert member preferably defines part of a substantially spherical surface whose radius of curvature preferably ranges from 4.75 mm to 8.5 mm, corresponding to nominal sizes of from 9.5 mm to 17 mm. In the 17 mm nominal size the radius of curvature of the substantially spherical surface is 8.5 mm, while this radius is 4.75 mm for a nominal size of 9.5 mm, 6 mm for a nominal size of 12 mm, and 7.25 mm for a nominal size of 14.4 mm.
Desirably the head portion of the metacarpal insert member is formed with a dorsal groove for receipt of an extensor tendon.
The stem portion of the metacarpal insert member may have a polished surface. In this case the stem portion can be free to move within the axial bore in the metacarpal plug member.
Alternatively the stem portion of the metacarpal insert may have a roughened surface. In this case movement of the stem portion can be restricted once received within the axial bore in the metacarpal plug member.
Yet again the stem portion can be a clip-fit within the axial bore in the metacarpal plug member.
Preferably the second fins are substantially oval in shape and may be about 0.6 mm thick.
In one preferred design the concave articulating surface is positioned asymmetrically with respect to the longitudinal axis of the phalangeal insert member so as to cause the center of rotation of the prosthesis to drop from the longitudinal axis of the phalangeal insert member.